Parkinson's Disease (PD) currently affects about 10 million people world-wide. PD is a highly specific degeneration of dopamine-containing cells of the substantia nigra of the midbrain. Degeneration of the substantia nigra in Parkinson's disease causes a dopamine deficiency in the striatum. Effective management of a patient with PD is possible in the first 5-7 years of treatment, after which time a series of often debilitating complications, together referred to as Late Motor Fluctuations (LMF) occur (Marsden and Parkes, Lancet II: 345-349, 1997). It is believed that treatment with levodopa, or L-dopa, the most effective antiparkinson drug, may facilitate or even promote the appearance of LMF. Dopamine agonists are employed as a treatment alternative, but they do not offer the same degree of symptomatic relief to patients as L-dopa does (Chase, Drugs, 55 (suppl. 1): 1-9, 1998).
Symptomatic therapies improve signs and symptoms without affecting the underlying disease state. Levodopa ((−)-L-alpha-amino-beta-(3,4-dihydroxybenzene) propanoic acid) increases dopamine concentration in the striatum, especially when its peripheral metabolism is inhibited by a peripheral decarboxylase inhibitor (PDI). Levodopa/PDI therapy is widely used for symptomatic therapy for Parkinson's disease, such as combinations with Ilevodopa, with carbidopa ((−)-L-alpha-hydrazino-alpha-methyl-beta-(3,4-dihydroxybenzene) propanoic acid monohydrate), such as SINEMET®; levodopa and controlled release carbidopa (SINEMET-CR®), levodopa and benserazide (MADOPAR®, Prolopa), levodopa plus controlled release benserazide (2-Amino-3-hydroxy-propionic acid N′-(2,3,4-trihydroxy-benzyl)-hydrazide), MADOPAR-HBS.
COMT (catechol-O-methyltransferase) inhibitors enhance levodopa treatment as they inhibit levodopa's metabolism, enhancing its bioavailability and thereby making more of the drug available in the synaptic cleft for a longer period of time. Examples of COMT inhibitors include tolcapone (3,4-dihydroxy-4′-methyl-5-nitrobenzophenone) and entacapone ((E)-2-cyano-3-(3,4-dihydroxy-5-nitrophenyl)-N,N-diethyl-2-propenamide).
Dopamine agonists provide symptomatic benefit by directly stimulating post-synaptic striatal dopamine receptors. Examples include bromocriptine ((5α)-2-Bromo-12′-hydroxy-2′-(1-methylethyl)-5′-(2-methylpropyl)ergotaman-3′,6′,18-trione), pergolide (8B-[(Methylthio)methyl]-6-propylergoline), ropinirole (4-[2-(Dipropylamino)ethyl]-1,3-dihydro-2H-indol-2-one), pramipexole ((S)-4,5,6,7-Tetrahydro-N6-propyl-2,6-benzothiazolediamine), lisuride (N′-[(8α)-9,10-didehydro-6-methylergolin-8-yl]-N,N-diethylurea), cabergoline ((8β)-N-[3-(Dimethylamino)propyl]-N-[(ethylamino)carbonyl]-6-(2-propenyl)ergoline-8-carboxamide), apomorphine ((6aR)-5,6,6a,7-Tetrahydro-6-methyl-4H-dibenzo[de,g]quinoline-10,11-diol), sumanirole (5-(methylamino)-5,6-dihydro-4H-imidazo{4,5,1-ij}quinolin-2(1H)-one), rotigotine ((−)(S)-5,6,7,8-tetrahydro-6-[propyl[2-(2-thienyl)ethyl]amino]-1-naphthol), talipexole (5,6,7,8-Tetrahydro-6-(2-propenyl)-4H-thiazolo[4,5-d]azepin-2-amine), and dihydroergocriptine (ergotaman-3′,6′,18-trione,9,10-dihydro-12′-hydroxy-2′-methyl-5′-(phenylmethyl) (5′α)). Dopamine agonists are effective as monotherapy early in the course of Parkinson's disease and as an adjunct to levodopa in more advanced stages. Unlike levodopa, dopamine agonists directly stimulate post-synaptic dopamine receptors. They do not undergo oxidative metabolism and are not thought to accelerate the disease process. In fact, animals fed a diet including pergolide were found to experience less age-related loss of dopamine neurons.
Amantidine (1-Aminotricyclo (3,3,1,13,7)decane) is an antiviral agent that was discovered by chance to have anti-parkinsonian activity. Its mechanism of action in PD has not been established, but it was originally believed to work by increasing dopamine release (Bailey et al., Arch. Int. Pharmacodyn. Ther., 216: 246-262, 1975). Patients who receive amantidine either as monotherapy or in combination with levodopa show improvement in akinesia, rigidity and tremor (Mann et al., Neurology, 21: 958-962, 1971; and Parkes et al., Lancet, 21: 1083-1086, 1971).
Other medications used in the treatment of Parkinson's disease include MAO-B inhibitors. Inhibition of L-dopa metabolism through inactivation of the monoamino oxidase type B (MAO-B) is an effective means of enhancing the efficacy of both endogenous residual dopamine and that exogenously derived from its precursor, L-dopa (Youdim and Finberg, Biochem Pharmacol. 41: 155-162, 1991). Selegiline (methyl-(1-methyl-2-phenyl-ethyl)-prop-2-ynyl-amine) is a MAO-B inhibitor. There is evidence that treatment with selegiline may slow down disease progression in PD by blocking the formation of free radicals derived from the oxidative metabolism of dopamine (Heikkila et al., Nature 311: 467-469, 1984; Mytilineou et al., J Neurochem., 68: 33-39, 1997). Another MAO-B inhibitor under development is rasagiline (N-propargyl-1-(R)aminoindan, TEVA Pharmaceutical Industries, Ltd.). Other examples of MAO B inhibitors include lazabemide (N-(2-Aminoethyl)-5-chloro-2-pyridinecarboxamide) and caroxazone (2-Oxo-2H-1,3-benzoxazine-3(4H)-acetamide).